When an egg has been cooked in hot water for 15 minutes or longer a
dark greenish color may be formed on the surface of the egg yolk. If the
egg is immersed in cold water immediately after cooking, the green color
is not produced or is less apparent than when the egg is left in the hot
w^ater to cool slowly.
The yolk contains most of the iron of the egg, about 85 times as much
as the white. According to Sherman, the sulfur content of the white is
slightly higher than that of the yolk, the white containing 0.214 per cent
and the yolk 0.208 per cent. The sulfur of the white is more labile and
more easily split off by heat than the sulfur of the yolk. Marlow and King
say that the sulfur in egg whites and yolk is organically bound and that
nearly all of it can be accounted for by the cystine and methionine sulfur.
Tinkler and Soar have shown that the green color is due to the forma-
tion of ferrous sulfide at the surface of the yolk. The white with prolonged
heating at high temperatures evolves considerable quantities of hydrogen
sulfide. The hydrogen sulfide combines with the iron of the yolk to form
ferrous sulfide which produces the green color. The amount of hydrogen
sulfide evolved depends on ( 1 ) the time of heating, (2) the temperature
reached, and (3) the reaction of the egg. In a short cooking period little
hydrogen sulfide is evolved. Likewise at a lower temperature a smaller
amount of hydrogen sulfide is formed. Tinkler and Soar have also show^n
that the formation of ferrous sulfide takes place very, very slowly until
the yolk reaches a temperature of 70°C. It is very seldom formed in eggs
cooked for 1 to 1^ hours at 70°C., or in eggs cooked 30 to 35 minutes
at 85°C. Thus both the temperature attained by all or a portion of the egg
white as well as the time held at this temperature influence the amount
of hydrogen sulfide formed. In addition, as the reaction becomes more
alkaline, the sulfur is split off more readily. Therefore, the extent of
deterioration of the egg also affects the amount of hydrogen sulfide formed.
This explains why some eggs have more ferrous sulfide formed than other
eggs when cooked at the same time and cooled in the same manner. Tinkler
and Soar have reported that the uncooked yolk is acid in reaction but upon
being heated above 70° becomes alkaline.
BAKED AND SOFT CUSTARDS 343
Effect of rapid cooling upon formation of ferrous sulfide in
cooked eggs. Hydrogen sulfide is a gas. When the egg is placed in cold
water immediately after cooking the lowering of the temperature at the
surface of the egg lowers the pressure there. One of the things learned in
connection with the gas laws is that the pressure increases with increasing
temperature. Since the white near the shell reaches a certain temperature
more rapidly than the white near the yolk, more hydrogen sulfide is split
off near the shell. But, since the temperature near the yolk is lower, hence
has less pressure than near the shell, the hydrogen sulfide diffuses through
the white towards the yolk. However, if the egg is placed immediately in
cold water after cooking, the hydrogen sulfide diffuses to the surface of
the egg, owing to the reduced pressure, but if left in hot water, or to cool
in the air, the gas does not diffuse so quickly to the surface and ferrous
sulfide is formed at the junction of the egg white and yolk.
Page 280
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When an egg has been cooked in hot water for 15 minutes or longer a
dark greenish color may be formed on the surface of the egg yolk. If the
egg is immersed in cold water immediately after cooking, the green color
is not produced or is less apparent than when the egg is left in the hot
w^ater to cool slowly.
The yolk contains most of the iron of the egg, about 85 times as much
as the white. According to Sherman, the sulfur content of the white is
slightly higher than that of the yolk, the white containing 0.214 per cent
and the yolk 0.208 per cent. The sulfur of the white is more labile and
more easily split off by heat than the sulfur of the yolk. Marlow and King
say that the sulfur in egg whites and yolk is organically bound and that
nearly all of it can be accounted for by the cystine and methionine sulfur.
Tinkler and Soar have shown that the green color is due to the forma-
tion of ferrous sulfide at the surface of the yolk. The white with prolonged
heating at high temperatures evolves considerable quantities of hydrogen
sulfide. The hydrogen sulfide combines with the iron of the yolk to form
ferrous sulfide which produces the green color. The amount of hydrogen
sulfide evolved depends on ( 1 ) the time of heating, (2) the temperature
reached, and (3) the reaction of the egg. In a short cooking period little
hydrogen sulfide is evolved. Likewise at a lower temperature a smaller
amount of hydrogen sulfide is formed. Tinkler and Soar have also show^n
that the formation of ferrous sulfide takes place very, very slowly until
the yolk reaches a temperature of 70°C. It is very seldom formed in eggs
cooked for 1 to 1^ hours at 70°C., or in eggs cooked 30 to 35 minutes
at 85°C. Thus both the temperature attained by all or a portion of the egg
white as well as the time held at this temperature influence the amount
of hydrogen sulfide formed. In addition, as the reaction becomes more
alkaline, the sulfur is split off more readily. Therefore, the extent of
deterioration of the egg also affects the amount of hydrogen sulfide formed.
This explains why some eggs have more ferrous sulfide formed than other
eggs when cooked at the same time and cooled in the same manner. Tinkler
and Soar have reported that the uncooked yolk is acid in reaction but upon
being heated above 70° becomes alkaline.
BAKED AND SOFT CUSTARDS 343
Effect of rapid cooling upon formation of ferrous sulfide in
cooked eggs. Hydrogen sulfide is a gas. When the egg is placed in cold
water immediately after cooking the lowering of the temperature at the
surface of the egg lowers the pressure there. One of the things learned in
connection with the gas laws is that the pressure increases with increasing
temperature. Since the white near the shell reaches a certain temperature
more rapidly than the white near the yolk, more hydrogen sulfide is split
off near the shell. But, since the temperature near the yolk is lower, hence
has less pressure than near the shell, the hydrogen sulfide diffuses through
the white towards the yolk. However, if the egg is placed immediately in
cold water after cooking, the hydrogen sulfide diffuses to the surface of
the egg, owing to the reduced pressure, but if left in hot water, or to cool
in the air, the gas does not diffuse so quickly to the surface and ferrous
sulfide is formed at the junction of the egg white and yolk.